In some of the conventional compressors using a thrust ball bearing, a rolling bearing is placed around a tubular extension portion in the upper part of the main bearing (see, for example, Patent Literature 1).
The conventional hermetic compressor disclosed in Patent Literature 1 will be described as follows with reference to the accompanied drawings. FIG. 16 is a longitudinal sectional view of the hermetic compressor of Patent Literature 1. FIG. 17 is an enlarged view of an essential part of a thrust ball bearing in the compressor. FIG. 18 is a perspective view of a support member in the compressor.
In FIG. 16, the compressor includes airtight container 2 having lubricating oil 4 at its bottom. Airtight container 2 includes compressor body 6 resiliently supported by suspension springs 8.
Compressor body 6 includes motor element 10 and compression element 12 arranged above it. Motor element 10 includes stator 14 and rotor 16.
Compression element 12 includes shaft 18 and cylinder block 24. Shaft 18 includes main shaft 20 and eccentric shaft 22. Main shaft 20, to which rotor 16 is fixed, is rotatably supported by main bearing 26 of cylinder block 24. A load applied to eccentric shaft 22 is supported by the eccentric-shaft-side portions of main shaft 20 and main bearing 26 arranged below eccentric shaft 22 so as to form a cantilever bearing.
Shaft 18 has lubrication mechanism 28 including a spiral groove on the surface of main shaft 20.
Compression element 12 further includes piston 30, which reciprocates in cylinder 34 having a substantially cylindrical inner surface in cylinder block 24. Compression element 12 further includes connection portion 36 having holes at its ends. Into these holes are fitted piston pin 38 of piston 30 and eccentric shaft 22 so as to connect eccentric shaft 22 and piston 30.
Cylinder 34 and piston 30 form compression space 48 together with valve plate 46, which is arranged on the open end face of cylinder 34. Valve plate 46 is covered with fixed cylinder head 50.
Cylinder head 50 is equipped with intake muffler 52, which is molded with a resin such as PBT (polybutylene terephthalate) in such a manner as to have a sound absorbing space inside.
The following is a description of a thrust ball bearing. In FIG. 17, main bearing 26 includes thrust face 60 and tubular extension portion 62. Thrust face 60 is a planar portion perpendicular to the central axis. Tubular extension portion 62 extends upward beyond thrust face 60 and has an inner surface facing main shaft 20.
Thrust ball bearing 76, which includes upper race 64, balls 66 held by holder 68, lower race 70, and support member 72, is formed on the outer-diameter side of tubular extension portion 62.
Upper and lower races 64 and 70 are annular metal plates each having parallel top and bottom sides. Holder 68 is annular in shape and has a plurality of holes in the circumferential direction in which balls 66 are held rotably.
As shown in FIG. 18, support member 72 includes an annular metal plate having downside projections 72a and 72b and upside projections 72c and 72d. These projections are formed of curved surfaces having the same radius, and arranged in such a manner that the line connecting the peaks of downside projections 72a and 72b and the line connecting the peaks of upside projections 72c and 72d are at right angles to each other.
On thrust face 60 are support member 72, lower race 70, balls 66, and upper race 64 arranged in contact with each other in this order. On the top surface of upper race 64, there is placed flange 74 of shaft 18. In support member 72, downside projections 72a and 72b are in line contact with thrust face 60, and upside projections 72c and 72d are in line contact with lower race 70.
The compressor having the above-described structure operates as follows. When electric power is supplied to motor element 10, stator 14 generates a rotating magnetic field, which allows rotor 16 to rotate with main shaft 20. The rotation of main shaft 20 makes eccentric shaft 22 perform eccentric rotation, which is transmitted to piston 30 via connection portion 36, allowing piston 30 to reciprocate in cylinder 34.
A refrigerant returned from a refrigeration cycle (not shown) outside airtight container 2 is introduced into compression space 48 via intake muffler 52, compressed by piston 30 therein, and sent from airtight container 2 to the refrigeration cycle (not shown).
The bottom of shaft 18 is soaked in lubricating oil 4, so that the rotation of shaft 18 allows lubricating oil 4 to be supplied to each unit of compression element 12 so as to lubricate the sliding part by lubrication mechanism 28.
The following is a description of thrust ball bearing 76. Thrust ball bearing 76 is a rolling bearing in which balls 66 are made to roll while being in point contact with upper and lower races 64 and 70. The rolling bearing can rotate while supporting the vertical load such as the weights of shaft 18 and rotor 16. Thrust rolling bearings have been increasingly used in recent years to achieve efficiency improvement because of less friction than generally-used thrust slide bearings.
In the cantilever bearing as shown in Patent Literature 1, however, when a large external force such as vibration during transportation is applied to the hermetic compressor, thrust ball bearing 76 is subjected to a large load, causing plastic deformation such as sinking in the contact area between balls 66 and upper and lower races 64 and 70. The deformation adversely affects the efficiency, noise level, and reliability.
Another example of thrust ball bearing 76 shown in FIG. 19 is disclosed in Patent Literature 2. FIG. 19 is an exploded perspective view of a thrust ball bearing of another conventional hermetic compressor, which is disclosed in Patent Literature 2. The overall structure of this hermetic compressor will be described with reference to FIG. 16 for convenience.
In FIGS. 16 and 19, on the main shaft 20 side between main shaft 20 and eccentric shaft 22 of shaft 18, there is provided an upper race seating surface (not shown). The upper race seating surface is annular in shape and substantially perpendicular to the central axis of main shaft 20. On the upper end of main bearing 26, there is provided lower race seating surface 80, which is annular in shape and substantially perpendicular to the central axis of main bearing 26. Between the upper race seating surface and lower race seating surface 80, thrust ball bearing 76 is provided which includes balls 66, and upper and lower races 64 and 70 in order to support shaft 18. Upper and lower races 64 and 70 are planar.
Thrust ball bearing 76 having the above-described structure supports the weights of shaft 18 and rotor 16 in the same manner as thrust ball bearing 76 in Patent Literature 1. The rotation of shaft 18 is made smooth by balls 66 rolling between upper and lower races 64 and 70.
During the rotation, upper race 64 rotates together with the upper race seating surface in contact therewith, and lower race 70 is at a standstill in contact with lower race seating surface 80. Using this thrust ball bearing 76 can reduce the loss of the thrust bearing because the torque to rotate shaft 18 is smaller than in a thrust slide bearing. As a result, motor element 10 requires less power, making the hermetic compressor efficient.
In the conventional structure of Patent Literature 2, however, compressors designed to meet the same specification may have variations in noise level and efficiency. A dismantling investigation of a hermetic compressor having a particularly high noise level has revealed that part of upper and lower races 64 and 70 is about to be peeled.
Such peeling is found to be caused as follows. When a compressive load is applied to piston 30 in the compression stroke, the compressive load is also applied to eccentric shaft 22 of shaft 18, which is connected to connection portion 36 via piston pin 38. As a result, shaft 18 is inclined in the clearance between main shaft 20 and main bearing 26 in cylinder block 24.
This prevents upper and lower races 64 and 70 from being parallel with each other, so that the space to hold balls 66 becomes non-uniform, causing the load to be concentrated only on balls 66 passing through a narrow portion of the space, instead of being applied evenly to all balls 66. As a result, balls 66, and upper and lower races 64 and 70 are subjected to excessive repeated stress, and hence, to damage such as fatigue peeling. This may lead to an increase in the noise level, thereby decreasing the efficiency and reliability.    Patent Literature 1: Japanese Translation of PCT Publication No. 2005-500476    Patent Literature 2: Japanese Patent Unexamined Publication No. 2005-127305